If you have several opportunities to update, you can do it all at once using a simple view. Create a view of opportunities that includes the field Process.
Make sure the results include at least one opportunity that is using the new process. This way, you’ll be able to clearly see the ID for the new process vs. the old.
Next, you’ll want to export the results to Excel or edit directly in Excel Online. Copy the ID of the new process over the ID of the old process.
In Excel Online before changing the Process ID:
In Excel Online after changing the Process ID:
Then import your changes and all of the opportunities will now be using the new process!